The cellular response to ischemia and reperfusion and to anoxia and reoxygenation will be examined using a preparation of isolated adult rat heart cells (myocytes). A myocyte model for ischemia will be developed using concentrated cell suspensions incubated in the absence of 02. Cellular alterations under conditions that simulate ischemia will be determined as a function of time and compared with those produced in anoxic cells. The effect of diluting the ischemic cells into aerobic media and of reaerating the anoxic cells will be assessed. A comprehensive picture of the relationship between the decline in ATP and creatine phosphate, the loss of total adenine nucleotide (AN) and changes in cellular and mitochrondrial nucleotide profiles, changes in respiratory and glycolytic capabilities, morphological alterations, changes in cellular ions and pH, and other variables will be established. Loss of sarcolemmal integrity or the hypercontracture of the normally rod-shaped myocytes into characteristic round forms with distorted morphology will be used as indicators of irreversible injury. The pathways for degradation of AN in anaerobic and ischemic myocytes will be established and conditions for optimal restoration of the components of this pool sought. The possibility that AN depletion in and of itself is a fatal lesion in myocytes will be examined, as will the effects of alterations in cellular lipid metabolism and oxygen radical damage. Interventions that may change the course of key cellular alterations will be identified. These include increasing the buffer capacity of the suspending medium to minimize alterations dependent on H+ accumulation or additions that slow or prevent the depletion of the AN pool. The effect of such interventions on the alterations associated with ischemic or anoxic cell damage will be assessed and used to identify the crucial events associated with the transition from reversible to irreversible injury.